The present invention relates to a battery system, and especially to a battery system composed of a plurality of lithium battery modules connected to each other, for use in an electric vehicle.
The lithium battery has recently received considerable attention as a light-weight, high power-density battery with a large capacity, and which is suitable for use in an electric vehicle. However, since lithium batteries have a low tolerance level against over-charging and over-discharging, the misuse of a lithium battery may cause a fire or an explosion. Therefore, it is necessary to closely monitor and control the terminal voltage of a lithium battery.
A conventional battery system composed of a plurality of modules serially connected to each other, each module including cell batteries serially connected to each other, is disclosed, for example, in a paper "Advanced Battery System for Electric Vehicle," EVS-13, Osaka, Japan. Each module is controlled in this battery system by a cell controller. A by-pass circuit is provided in each cell, and the cell controller controls each module so that any charging variation among the cells in each module is suppressed. The total battery system is controlled by a battery controller, and each cell controller sends signals to the battery controller.
As mentioned above, although the conventional battery system controls the charging-balance among the cells in each module, the system does not take the charging variation among the modules into consideration. However, in a battery system composed of many modules serially connected to each other, if there is a charging variation among the modules, the terminal voltage of only some of the modules attains the rated terminal voltage, and the terminal voltage of the rest of the modules remains under the rated terminal voltage, Accordingly, the full capacity of the battery system cannot be achieved. Furthermore, if there exists a charging variation among the modules, this also causes a variation in the length of their life cycles.